Resolve #1034 Migrate from appdirs to platformdirs

[lombad]

Describe the changes you have made:

Removed the deprecated appdirs library. Switched from appdirs to platformdirs in all source files.

Reference any relevant issues (e.g. "Fixes #000"):

Fixes #1034

Pre-Submission Checklist (optional but appreciated):

• [ ] I have included relevant documentation updates (stored in /docs)
• [x] I have read docs/CONTRIBUTING.md
• [x] I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• [ ] Tested on Windows
• [ ] Tested on MacOS
• [x] Tested on Linux